1. Field of the Invention
The present invention generally relates to a radio antenna. More particularly, the present invention relates to an improved radio antenna that is compact, mountable to a conductive surface, and having nearly constant gain over a hemisphere of solid angle so that it is essentially omni-directional when located near the surface of the earth.
2. Related Art
It is generally known that antenna performance is dependent upon the size and shape of the constituent antenna elements as well as the relationship between various antenna physical parameters (e.g., the length for a linear antenna and diameter for a loop antenna) and the wavelength of the signal. These relationships determine several antenna operational parameters, including input impedance, gain, and radiation pattern. In general, the minimum physical dimension for an operable antenna is on the order of a quarter wavelength of the operating frequency or some multiple thereof.
The rapid and wide spread growth and utilization of GPS and wireless communications and the evolution of the devices that support these systems has created a continued need for physically smaller, more efficient antennae that are capable of wide bandwidth operation, and multiple frequency-band operation. As the size of these devices shrink, the antennae used by the devices must shrink correspondingly. Thus physically small antennae operating in the frequency bands of interest and providing properties such as high gain and omni-directionality continue to be sought after.
One antenna commonly used in many applications today is the half-wavelength dipole antenna. The radiation pattern of this device is the familiar toroidal donut shape with most of the energy radiated uniformly in 360° of rotation perpendicular to the longitudinal axis of the dipole with energy decreasing with increasing angular elevation from the horizon. Antenna gain, therefore, is highest for a vertical dipole in a plane of the horizon and decreases with increasing angular elevation from the horizon. In order to efficiently detect systems such as GPS and cellular signals, it is desirable to have an antenna whose gain is nearly constant gain over a hemisphere of solid angle so that it is essentially omni-directional above the horizon for antennae located near the surface of the earth.